EP-A-0783986 describes a roll control system in which hydraulic actuators are powered to provide active roll control for the vehicle. The actuators generate torsion in the vehicle's torsion bars. WO-A-02/83439 describes an active roll control system having two hydraulic actuators attached to each torsion bar. EP-A-1103395 discloses a vehicle roll control system in which a pair of directional valves and a pressure control valve are used to control the movement of the piston of hydraulic actuators associated with the front and rear axles of a motor vehicle. WO-A-03/093041 discloses a vehicle roll control system in which a pair of pressure control valves and a directional valve are used to control the movement of the piston of hydraulic actuators associated with the front and rear axles of a motor vehicle. In both cases, each hydraulic actuator has a first fluid chamber positioned on one side of the piston, and a second fluid chamber positioned on the other side of the piston. The first fluid chambers of the front and rear hydraulic actuators receive hydraulic fluid at substantially the same pressure; and the second fluid chambers of the front and rear hydraulic actuators receive hydraulic fluid at substantially the same pressure. WO-A-2005/108128 discloses a roll control system in which the control means for the hydraulic circuit is capable of providing fluid pressure to the first fluid chamber of the front hydraulic actuator which is different from the fluid pressure provided to the first fluid chamber of the rear hydraulic actuator; and/or is capable of providing fluid pressure to the second fluid chamber of the front hydraulic actuator which is different from the fluid pressure provided to second fluid chamber of the rear hydraulic actuator.